1. Field of the Invention
The present invention relates to a subscriber station, a time division multiplexing system and a transmission timing control method.
2. Description of the Related Art
In Document 1 (IEEE Standard for Local and metropolitan area networks Part 16: “Air Interface for Fixed and Mobile Broadband Wireless Access Systems Amendment 2”, IEEE Std 802.16e-2005 and IEEE Std 802.16-2004/Cor 1-2005, 2006.), a standard for high-speed wireless data communication (IEEE 802.16) has been defined.
In a physical layer in IEEE 802.16, single carrier, OFDM (orthogonal frequency division multiplexing), OFDMA (orthogonal frequency division multiple access) and the like are supported as modulation schemes. Hereinafter, a wireless data communication method compliant with IEEE 802.16 will be described. In addition, it is assumed that the modulation scheme is OFDMA.
In IEEE 802.16, a PtoMP (point-to-multipoint) scheme has been prescribed as one of the communication modes. In the PtoMP scheme, a base station (hereinafter referred to as “BS”) performs scheduling of transmission/reception timings of all subscriber stations (hereinafter referred to as “SS”) under the BS, which enables improving communication efficiency and guarantee QoS (Quality of Service).
FIG. 1 is a diagram showing a configuration example of an OFDMA frame in the PtoMP scheme of the related art. In the OFDMA frame, a Preamble and an FCH followed by a Down Link Burst are stored. Schedule information for scheduling the transmission/reception timing of the subscriber station (scheduling information for each of Down Link and Up Link) exists in a MAP message included in a broadcast message field within this Down Link Burst.
The scheduling information includes down link slot information and up link slot information to be assigned to each SS. Based on this slot information, the SS can know a timing at which data arrives at the SS itself (reception timing) and a timing at which the SS itself may transmit the data (transmission timing).
Moreover, if the SS connects to a network, the SS performs a process referred to as “ranging” for adjusting a transmission/reception timing and transmission power of the BS and the like. In addition, the SS periodically performs ranging even during connecting to the network.
If the SS connects to the network, the SS performs ranging in an initial ranging period assigned by the MAP message from the BS. In addition, this initial ranging period is assigned by a contention scheme. Moreover, ranging in the initial ranging period is referred to as “initial ranging”.
FIG. 2 is a sequence diagram for explaining operations of the BS and the SS when the initial ranging is performed in the related art.
The SS transmits a CDMA Code (code division multiple access code: ranging request) message to the BS in the initial ranging period (step 701).
When the BS has received the CDMA Code message, if adjustment of the transmission timing, the transmission power, a frequency and the like is required for the SS, the BS transmits information thereof to the SS by means of RNG-RSP (ranging response) including a ranging continued notification (Ranging Status=continue in the RNG-RSP) (step 702).
When the SS has received the RNG-RSP, the SS adjusts the transmission timing, the transmission power, the frequency and the like based on the RNG-RSP. Subsequently, the SS transmits the CDMA Code message to the BS again in the initial ranging period assigned to the SS by the BS (step 703).
Moreover, if adjustment is not required for the SS, the BS transmits the RNG-RSP including a ranging success notification (Ranging Status=success in the RNG-RSP) to the SS (step 704).
When the SS receives the RNG-RSP including the ranging success notification, the initial ranging is completed.
The related art of a communication system for performing such ranging is described in, for example, Document 2 (Japanese Patent Laid-Open No. 2002-94606), Document 3 (Japanese Patent Laid-Open No. 06-013998), Document 4 (Japanese Patent Laid-Open No. 2006-303802), Document 5 (Japanese Patent Laid-Open No. 11-331228), Document 6 (National Publication of International Patent Application No. 2001-524268), and Document 7 (National Publication of International Patent Application No. 1999-510667).
However, if the size of a cell of the BS becomes large, and PtoMP communication over great distances or PtoP (Point to Point) communication over great distances is executed, there will be a problem in which the initial ranging may fail.
Hereinafter, this problem will be described in detail. FIG. 3 is a configuration diagram showing an example of a communication system in which this problem occurs. In addition, in FIG. 3, a time division multiplexing system is used as the communication system.
In FIG. 3, the time division multiplexing system includes BS 801 and SS 802 to 804. BS 801 mutually communicates with each of SS 802 to 804. Moreover, distances between BS 801 and respective SS 802 to 804 are different from one another. Hereinafter, it is assumed that the distance between BS 801 and SS 802 is A, the distance between BS 801 and SS 803 is B, and the distance between BS 801 and SS 804 is C. Moreover, it is assumed that A<B<C is satisfied.
Since the distances between BS 801 and respective SS 802 to 804 are different from one another, propagation distances of the CDMA Code messages transmitted to BS 801 by respective SS 802 to 804 are different from one another. Thus, propagation delay times between BS 801 and respective SS 802 to 804 are different from one another. Therefore, the optimum transmission timings for the CDMA Code messages in respective SS 802 to 804 are different from one another.
FIG. 4 is an explanatory diagram for explaining an example of the transmission timing of the SS of the related art. In FIG. 4, it is assumed that a distance between BS 201 and SS 202 is significantly large.
When time 204 which has been previously defined has elapsed since the time when SS 202 received beginning 203 of Down Link data from BS 201, SS 202 transmits CDMA Code message 206 to BS 201. Here, since the distance between BS 201 and SS 202 is significantly large, CDMA Code message 206 arrives at BS 201 at time 208 beyond time 209 in which BS 201 can receive the message.
As seen from this example, when the communication (the PtoMP communication or the PtoP communication) over great distances is executed, even if the SS has transmitted the CDMA Code message in the initial ranging period, the CDMA Code message may not arrive at the BS at the time at which the BS can receive the message. If the CDMA Code does not arrive at the BS in the time in which the BS can receive the message, the initial ranging fails.
Here, it is conceivable that this problem can be solved if the transmission timing for the CDMA Code message in the SS is changed depending on the distance between the SS and the BS. However, the BS is required to precisely comprehend the distance to the SS, thus taking a lot of trouble.
Moreover, in IEEE 802.16 described in Document 1, a technique has been proposed in which failure of the initial ranging can be prevented without changing the transmission timing for the CDMA Code message depending on the distance between the SS and the BS. In this technique, a redundancy referred to as “Guard interval” is added to an OFDM symbol. Thereby, even if the time of receiving the CDMA Code message in the BS has been shifted from the time at which the CDMA Code message can be received, using a Guard interval time secured at the Guard interval, the shift can be absorbed. In other words, even if the time of receiving the CDMA Code message has been shifted from the time in which the CDMA Code message can be received, using Guard interval time, the CDMA Code message can be received. Thus, it is possible to prevent failure of the initial ranging.
Here, in order to absorb a large shift, it is necessary to set the Guard interval to be large and have a long Guard interval time. However, if the Guard interval is increased, a data area for one OFDM symbol becomes narrow. This is because a length of the OFDM symbol having the data area and the Guard interval has been previously defined. Therefore, a method of increasing the Guard interval cannot be said to be a preferred method since the data transmission amount for one OFDM symbol is decreased. Moreover, primarily, in a communication system in which the Guard interval cannot be set, the shift cannot be absorbed.
Document 2 describes a communication subscriber station apparatus capable of receiving the CDMA Code without setting the Guard interval in the case where communication over great distances is performed.
This communication subscriber station apparatus measures receiving electric field strength from a base station, and based on the measured received electric field strength, an initial value of a transmission timing with respect to the base station is determined. Then, the communication subscriber station apparatus transmits a signal while shifting the transmission timing, and thereby searches for a transmission timing at which the base station can receive the signal.
Thereby, it is possible to receive the CDMA Code message without the Guard interval being set.
However, the communication subscriber station apparatus described in Document 2 has to include means for measuring the receiving electric field strength and means for obtaining the initial value of the transmission timing based on the receiving electric field strength, thereby causing a problem the configuration of the apparatus becomes complicated.
Moreover, in the communication system performing the ranging, if communication over great distances is executed, there is another problem, as follows, in addition to the above described problem in which the initial ranging fails. In other words, there is a problem in which the reception time of receiving the data from the BS and the transmission time of transmitting the data to the BS overlap in the SS.
FIG. 5 is an explanatory diagram showing an example in which the reception time and the transmission time overlap in the time division multiplexing system of the related art. In FIG. 5, it is assumed that the distance between BS 301 and SS 302 is larger than distances between BS 301 and other SS (SS 311 and 312).
In FIG. 5, reference numeral D1 (303) denotes a Down Link data area with respect to SS 311, reference numeral D2 (304) denotes a Down Link data area with respect to SS 312, and reference numeral D3 (305) denotes a Down Link data area with respect to SS 302.
If the distance between BS 301 and SS 302 is large, as shown in FIG. 5, at SS 302, the transmission timing for Up Link data may occur by the time that reception of data area D3 (305) from BS 301 has been completed.
Specifically, BS 301 does not consider which position within a Down Link frame the Down Link data area with respect to SS 302 is mapped at. Thus, if data with respect to SS 302 has been mapped at the portion of the behind the Down Link frame (data area D3 (305)), the reception time for data area D3 (305) and the transmission time for an Up Link frame overlap.
Thus, there has been a problem in which data cannot be correctly received.
Document 3 describes a method of assigning transmission time slots in which the data areas within the Down Link frame and the Up Link frame are assigned so that such overlap may be prevented.
However, since Document 3 does not include the above described configuration for shifting the transmission timing, there is a case where the CDMA Code message cannot be received.
Furthermore, Documents 1 and 4 to 7 do not describe means for solving the above described problem in which the CDMA Code cannot be received.